Designing Deep-UV Birefringent Crystals by Cation Regulation.

Two new alkaline earth metal fluorooxoborates, CaB4 O6 F2 and SrB4 O6 F2 , have been synthesized and characterized. The title compounds are isostructural and crystallize in centrosymmetric space group P1‾ (No. 2). Structurally, the fundamental building blocks, B4 O8 F2 groups, are connected by B-O bonds to form [B4 O6 F2 ]∞ layers, which stack parallelly along the c axis. In addition, BaB4 O6 F2 , a known fluorooxoborate with different crystal structure, was also synthesized and characterized for comparison. The cation size influence on the structure of this MB4 O6 F2 (M=Ca, Sr, Ba) family is discussed in detail. The diffuse-reflectance spectroscopy measurements indicate that the cutoff edges of title compounds are below 190 nm. Moreover, first-principle calculation results demonstrate that MB4 O6 F2 (M=Ca, Sr, Ba) have large birefringence of 0.091, 0.089, and 0.085 at 1064 nm. Both experimental and theoretical studies suggest that MB4 O6 F2 (M=Ca, Sr, Ba) are promising birefringent materials for deep-UV applications.